Accurate image-guidance plays a critical role in improving current proton therapy (PT) protocols. We aim to assess the clinical benefits of integrating MRI and PT and compare the toxicities with the alternative available image-guided methods for treatment of liver cancer. Ten patients with liver tumors who underwent serial MRI scans were retrospectively selected. The reduction of PTV margins due to real-time and offline MRI-guidance were quantified compared to conventional IGRT (CBCT-guided). Proton treatment plans were created for: (1) CBCT-guided, (2) offline MRI-guided, (3) real-time MRI-guided (MRiPT), and (4) MR-linac (VMAT plan with reduced margins) scenarios. Normal tissue complication probability (NTCP) of uninvolved liver was compared to assess the clinical impact of margin reduction. The resulting residual margin for MRiPT was 3 mm isotropic on MRI-defined GTV. Uninvolved liver NTCP was significantly decreased for Child-Pugh score changes endpoint for MRiPT compared to CBCT-guided (up to 48%), Offline MRI-guided (up to 13%) and MR-linac (up to 30%). In conclusion, the clinical benefits of MRiPT are potentially significant and exceed that of currently available IGRT techniques for liver tumors.